Anti-fog film, method of packaging products having high moisture content and products thus packaged

ABSTRACT

A multi-layer heat-shrinkable film endowed with high anti-fog properties, and having a barrier layer (1), an adhesive layer (2, 2&#39;), and an adhesive layer (3, 3&#39;), wherein at least one external layer (3, 3&#39;) is made of a polyolefinic polymer. Said anti-fog properties are obtained by incorporating in said film from 1 to 5% by weight of a composition comprising (i) a glycerol mono-ester of a saturated or unsaturated fatty acid having from 8 to 20 carbons, (ii) a glycerol di-ester of a saturated or unsaturated fatty acid having from 8 to 20 carbons, and (iii at least one polyoxyethylene ether of a fatty alcohol having from 8 to 20 carbons.

DESCRIPTION

The present invention relates to a film endowed with high anti-fogproperties, a method for packaging products having a high moisturecontent and products thus packaged.

In particular, the invention relates to a thermoplastic polyolefin filmwith high anti-fog properties wherein said properties are obtained byincorporation of a composition comprising the following additives (i) amono-ester of glycerol with a saturated or unsaturated fatty acid, (ii)a di-ester of glycerol with a saturated or unsaturated fatty acid, (iii)at least one ether of a polyoxyethylene with a fatty alcohol.

With the term "anticondensation" (or, to use the English term moreimmediately evident to the person skilled in the art, "anti-fog"), it ismeant the capability of the film to prevent water vapour from condensingonto its surface in the form of small water drops redistributing them inthe form of a continuous film of water in a very thin layer.

In fact, it is common practice in supermarkets to package perishablefood products, such as fresh meat cuts, vegetables, cooked food,deep-frozen food and the like, in trays of different materials andshapes, wrapped in a transparent, heat-shrinking film, through which itis very important for the product to be clearly visible. With films thathave no anti-fog properties, in the case for instance ofeasily-perishable food products, having a high moisture content, therewill be a rapid condensation of water vapour and the formation ofdroplets on the internal surface of the film with the consequent loss oftransparency of the packaging. This occurs especially when, afterpackaging, the product is subjected to sharp and substantial changes intemperature, such as, for example, when it is subjected to refrigerationor deep-freeze cooling.

This phenomenon occurs to a greater or to a lesser extent depending onthe type of polymer or of the blend of polymers of which the film ismade or, in the case of a multi-layer film, of which is made the surfacelayer facing the content of the tray. More particulararly, thisphenomenon occurs to a greater or to a lesser extent depending on thegreater or lesser hydrophobic properties of said polymer or said blend.Indeed, even if this phenomenon is common to all types of film that donot contain suitable anti-fog additives, it is especially important whensaid polymer or said blends are polyolefinic in nature.

Several very large families of potentially anti-fog compounds, as wellas their use in film compositions used to produce film for food-wrappinguses, have been studied for a number of years.

Examples of such families comprise sorbitan esters of aliphaticcarboxylic acids, glycerol esters of aliphatic carboxylic acids, estersof other polyhydric alcohols with aliphatic carboxylic acids,polyoxyethylene compounds, such as the polyoxyethylene sorbitan estersof aliphatic carboxylic acids and polyoxyethylene ethers of higheraliphatic alcohols.

The literature is rich in papers relating to investigations to findwhich are the specific members of the abovementioned families that,alone or in admixture among them, impart the maximum anti-fog propertiesto one or to the other type of film-forming composition but generalcriteria suitable for predicting which is the most effective anti-fogcomposition for each type of film-forming composition do not yet exist.In this field, experiments continue to offer new and unexpected results.

The publication--even recent--of various patent applications concerninganti-fog compositions made of members belonging to the abovementionedfamilies is a clear indication of the fact that an optimunm solution ofthe problem has not yet been found and that the search for newcompositions capable of improving the results attained thus far, interms of anti-fog properties, or of guaranteeing their reproducibilityunder increasingly severe conditions or--again--of simplifying theprocess of obtaining films having the desired anti-fog properties, isstill open.

In particular, the need is still very keenly felt of a compositioncapable of imparting the maximum possible anti-fog properties topolyolefinic layers of irradiated heat-shrinkable films.

Irradiation is a cross-linking treatment of the polymeric materialperformed by bombarding a film with accelerated electrons to the aim ofincreasing its structural resistance, to enhance its optical propertiesand the properties at high temperature.

It is generally carried out before the possible stretching step and itis typically made in range of from about 1 to about 13 MRad.

It is a highly energetic process capable of degrading the smaller andweaker organic molecules, such as is typically the case with themolecules of the various additives, thus including the anti-fogadditives.

It is therefor a first object of the present invention to provide amono- or multi-layer thermoplastic film having at least one externallayer of a polyolefinic polymer endowed with very high anti-fogproperties which are immediately manifest and last for a long time.

A second object of this invention is to provide a heat-shrinkable,thermoplastic film that is capable of maintaining such outstandingproperties even after irradiation.

A third object of the present invention is to provide a method forpackaging a product having a high moisture content with aheat-shrinkable, thermoplastic film, having the abovementionedcharacteristics, wherein said film is not in contact with the packagedproduct.

A further object of the present invention is to provide a packagecomprising a product having a high moisture content placed in a suitabletray and wrapped by a shrunk film having the abovementioned properties.

Such objects have been attained with the present invention that, as afirst object, provides a mono- or multi-layer heat-shrinkable film,endowed with high anti-fog properties wherein at least one externallayer is made of a polyolefinic polymer, characterized in that saidanti-fog properties are obtained incorporating in said film from about 1to about 5% by weight of a composition comprising

(i) a glycerol mono-ester of a saturated or unsaturated fatty acidhaving from 8 to 20 C,

(ii) a glycerol di-ester of a saturated or unsaturated fatty acid havingfrom 8 to 20 C, and

(iii) at least a polyoxyethylene ether of a fatty alcohol having from 8to 20 C,

wherein the weight ratio of the combined esters (i) and (ii), to theweight of the ether (iii) is in the range of from about 2:1 to about1:4.

In general, the percent ratio of the weight of the mono-ester (i) to thetotal weight of both mono-ester (i) and the di-ester (ii) may range fromabout 0.1 to about 0.9; it shall range preferably from about 0.25 toabout 0.75 and even more preferably from about 0.4 to about 0.6.

Preferably, the saturated or unsaturated fatty acid has from 12 to 20 Cand when it is unsaturated it may have one or more unsaturations.Examples of suitable fatty acids are lauryl acid, palmitic acid, stearicacid, oleic acid, linoleic acid, linolenic acid and the like. Preferredacids are stearic acid and oleic acid and more preferred is oleic acid.According to a particularly preferred embodiment the mono-ester and thedi-ester are both derived from the same fatty acid.

Generally the polyoxyethylene ether is a mixture of reaction products ofethylene oxide with one or more fatty alcohols having from 8 to 20 Cand, preferably, of products obtained by reacting from 2 to 5 moles ofethylene oxide per mole of fatty alcohol. Preferably, the fatty alcoholhas from 8 to 18 C. Examples of suitable fatty alcohols are laurylalcohol, myristyl alcohol, palmityl alcohol and stearyl alcohol andtheir mixtures. A particularly preferred mixture is that of laurylalcohol with myristyl alcohol, wherein the amount of lauryl alcoholgreater than about 50%.

In this present description and in the claims,

the term "polymer" comprises, but is not limited to, the homopolymers,the copolymers, the terpolymers, the block polymers, the blends thereofand their possible modified forms, as well as any possible symmetricalor asymmetrical structure consistent with their nature, e.g.,syndiotactic, isotactic, atactic, thermoplastic and film-forming;

the term "polyolefin" means any thermoplastic, film-forming polymerobtained by polymerization or copolymerization of olefins with othermonomers, wherein the olefin units are present in amounts greater thanthe other monomers. Non-limitative examples of suitable polyoelfins are:low density polyethylene (LDPE), linear low density polyethylene(LLDPE), Linear medium density polyethylene (LMDPE), Linear very-lowdensity polyethylene (VMDPE), Linear ultra-low density polyethylene(UMDPE), high density polyethylene (HDPE), polypropylene (PP),syndiotactic polypropylene (SPP), propylene/ethylene copolymers,propylene/alpha-olefin copolymers or terpolymers, the so-calledpolyethylene interpolymers, that is to say the copolymers of ethylenewith alpha-olefins characterized by a narrow distribution of themolecular weights and obtained by means of polymerization withmetalocene catalysts (see EP-A-416,815 and U.S. Pat. No. 4,306,041), thecopolymers of ethylene with other monomers, in particular with vinylacetate, (EVA), wherein the ethylene units are present in an amountgreater than those of vinyl acetate, and the blends thereof, in anyproportion;

the term "LLDPE" means the copolymers of ethylene with one or morecomonomers, preferably selected among the alpha-olefins, such as butene,octene, hexene, pentene, methylpentene, etc., wherein the molecules ofthese copolymers comprise long chains with few branches. Generally LLDPEhas a density ranging from about 0.916 g/cc to about 0.925 g/cc;

the term "LMDPE" means ethylene copolymers as defined above for LLDPE,characterized by a density usually ranging from about 0.926 g/cc toabout 0.941 g/cc;

the term "VLDPE" means linear ethylene copolymers characterized by adensity usually ranging from about 0.910 g/cc to about 0.860 g/cc;

the term "VLDPE" means linear ethylene copolymers characterized by adensity lower than about 0.860 g/cc;

the term "EVA" identifies a copolymer formed by monomers of ethylene andvinyl acetate wherein the units derived from the ethylene monomer in thecopolymer are present in a larger amount, preferably in an amountranging from about 60 to about 98% by weight on the total weight of thecopolymer.

The additives (i), (ii) and (iii) may be incorporated in the polymer orin the blend of film-forming polymers before or during extrusion.

In particular, such additives can be mixed intimately with the polymersin which they are to be incorporated, when the extruder is loaded. Or,this step of intimate mixing may be carried out beforehand, by means ofthe passage of the polymer and of the anti-fog additives through asingle- or twin-screw extruder and the pelletization of the thusobtained product that will later be loaded as it is in the extruder.Alternatively, with this latter system, it is possible to prepare aso-called "masterbatch", in practice a polymer having a highconcentration of anti-fog additives that will later be mixed with thepolymer in which said composition is to be incorporated at the moment ofextrusion and in an appropriate quantity to provide the desiredpercentage of anti-fog additives.

In this case the base polymer used for the preparation of themasterbatch can be the same as that in which it is desired toincorporate the anti-fog additives or it can also be different as longas it is compatible with the latter.

In the case of multi-layer films, the anti-fog additives of the presentinvention will not necessarily have to be incorporated in the polymer orin the blend of polymers forming the external layer, where the term"external layer" means that surface layer of the film that will laterbecome the internal surface--facing the product having a high moisturecontent of the final package. Should the layer adjacent to the externallayer allow the migration of the anti-fog additives (i), (ii) and (iii)without interfering with them, it may be expedient and will in any casebe feasible, to incorporate said additives or part of them in such moreinternal layer.

A situation of this type takes place for sure when also the layeradjacent to the external one is a polyolefin layer as defined above. Inthe case of films having more than three layers, when their polymercomposition allows it, it is possible and it can be expedient toincorporate the additives (i), (ii) and (iii) or part of them in an evenmore internal layer or to split them in all the layers.

If, as frequently occurs, the film is symmetrical, the additives (i),(ii) and (iii) can be incorprated either only in the "external layer" asdefined above or, essentially for practical reasons, in both surfacelayers.

The total amount of the additives (i), (ii) and (iii) will be such as torepresent from about 1 to about 5% and, preferably, from about 1.25 toabout 3.5% by weight of the total weight of the film of this invention.

In particular, the percentage by weight of the additive (iii) on thetotal weight of the film of this invention will typically range fromabout 0.6 to about 2.9%, preferably from about 1 to about 2%, and evenmore preferably from about 1.2 to about 1.5%.

On the other hand, the percentage by weight of the sum of the additives(i) and (ii) will range from about 0.4 to about 2.1%, preferably fromabout 0.5 to about 1%, and even more preferably from about 0.6 to about0.8% on the total weight of the film of this invention.

Other additives can obviously be added to the film-forming compositionsto enhance their workability and/or other characteristics of the finalfilm.

Among these one can mention additives in common use such as the slip andthe antiblock agents; where the latter term means those substances thatreduce the tendency of the films to adhere to themselves or to othersurfaces.

Examples of commonly used substances which have this type of activitycomprise colloidal silica, finely subdivided silica and silicone. Theycan be incorporated in the film-forming compositions or, in some cases,atomized on the surface of the film.

Other additives, not explicitly exemplified here, such as for examplepossible pigments or colourants, fungicide agents, etc., can easily beadded, if required.

Any conventional technique can be used to produce films according to thepresent invention. In particular, either a round or a flat die may beused, quickly cooling the extruded tube or tape, respectively, and thenstretching (or "orienting") it in one or both directions (longitudinalor machine direction, and transversal direction) thus obtaining aheat-shrinkable (or "oriented") film. It is also possible to stabilizethe orientation of the stretched film by keeping the oriented film undersuitable temperature conditions.

It is also possible to obtain a non-heat-shrinkable film using, in theextrusion step, the so-called "hot blown" technique by means of whichthe stretching up to the desired thickness is performed directly on thespindle before cooling.

Preferred films according to the present invention are those barrier ornon-barrier films, wherein the external or the only layer comprises apolyolefin blend of at least two components wherein one is anethylene/vinyl acetate copolymer and the other or the others arecopolymers or terpolymers of ethylene with one or more alpha-olefins.

The films of this invention can have the structure disclosed in U.S.Pat. No. 4,532,189, U.S. Pat. No. 4,551,380, EP-B-87,080, EP-B-217,596,EP-B-236,099, EP-A-286,430, EP-A-435,498, and EP-A-561,428, the contentof which is incorporated herein by reference.

FIG. 1 represents a preferred embodiment of a film with five layersaccording to the present invention; the internal layer 1 is a barrierlayer preferably made from a blend of an ethylene/vinyl alcohol (EVOH)copolymer and a polyamide, the intermediate layers 2 and 2' are adhesivelayers preferably made from a modified low density polyethylene and thetwo layers 3 and 3' are two polyolefin layers. The term "ethylene/vinylalcohol copolymer" or "EVOH" is intended to comprise also the hydrolyzedor saponified ethylene/vinyl acetate copolymers and refers to a vinylalcohol copolymer having an ethylene comonomer, typically present in anamount of from 15 to 65% by moles, which may be obtained, for example,by hydrolyzation of an ethylene/vinyl acetate copolymer or by chemicalreaction of ethylene monomers with vinyl alcohol. When this product isobtained by hydrolyzation, the hydrolyzation degree is preferably higherthan about 50%, even more preferably at least 85%.

As used herein, the term "polyamide" means polymers having a highmolecular weight and amide bonds in their chain, and more specificallymeans synthetic polyamides such as the different types of nylon. Thisterm also comprises polyamide copolymers such as of nylon 6 and 12.

At least one of the two layers 3 and 3' contains the additives (i), (ii)and (iii) of this invention. When the abovementioned additives areincorporated only in the layer 3 or 3', in the packaging method of thispresent invention care will have to be taken that said layer is the onefacing the product having a high moisture content placed in the tray.This precaution shall not, on the other hand, be taken when theabovementioned additives will be incorporated both in 3 and in 3'.

It is a second object of the present invention to provide a method forpackaging a product having a high moisture content wherein said productis placed in a suitable tray, the tray is wrapped in a mono- ormulti-layer, heat-shrinkable, thermoplastic film, the film is sealed orclipped and then shrunk in a such a way that the surface of the externallayer facing said product having a high moisture content is not incontact with the same, characterized in that a film according to thepresent invention is used.

It is a further object of the present invention to provide a packagecomprising a product having a high moisture content, placed in asuitable tray and wrapped in a shrunk film not in contact with saidproduct, characterized in that a film according to the present inventionis used. As

As a non-limiting example the preparation will be hereinafter describedof an anti-fog film according to the present invention. It is understoodthat on the basis of the above description, of the example that followsand of his own knowledge, any person skilled in the art shall thus beable to practice the present invention in any of its embodiments evenwhen it is not described in detail.

EXAMPLE 1

An anti-fog, multi-layer, heat-shrinkable film is prepared from a blendcomprising 50% (w/w) of LLDPE, 25% (w/w) of LMDPE and 25% (w/w) of EVA(having a content of vinyl acetate of about 3.6% by weight). To thisblend there is added a mixture of glycerol mono- and di-oleate(ATMER™121--sold by ICI), a mixture of ethers of polyoxyethylene withfatty alcohols (C12-C14) (MERGITAL™ LM3 sold by Henkel) and conventionalantiblocking agents. The ratio by weight of ATMER™ to MERGITAl™ LM3 isabout 2:1 and their total content is about 2% of the total weight of thefilm.

This blend for the two external layers is coextruded with an internal(core) layer made of a blend of 90% of EVOH and 10% of a nylon 6/nylon12 copolymer and two intermediate adhesive layers made of a low densitypolyethylene modified with anhydride groups.

The film is obtained by "cast" extrusion, cross-linking by irradiationat about 4MRad, and finally by air-bubble orientation with a stretchingratio of about 3.5:1 in both directions.

In order to assess the anti-fog properties of the thus obtained film astandard procedure is used wherein film samples are employed tohand-package trays having a 0.5 l capacity and containing about 200 mlof tap water at a temperature of about 15° C., and the film is thenshrunk with hot air. These trays are then placed in a refrigerator at atemperature of from 2° to 4° C. and the formation of fog is monitored aswell as its possible disappearance after 5', 20', 2h, 24h, 2d, 4d, 6d,8d and 10d.

The results, at each time, are assessed on a scale from 1 to 5 with thefollowing meanings:

5=excellent--the bottom of the tray is clearly visible through the filmand there are no drops of water on the film itself;

4=good--the bottom of the tray is clearly visible but a few droplets ofwater are present in a limited quantity on the surface of the film;

3=acceptable--the bottom of the tray can be seen in spite of thepresence of a certain number of droplets on the surface of the film;

2=poor--it is difficult to see the bottom of the tray due to theconsiderable number of water droplets on the surface of the film;

1=very poor--it is impossible to see the bottom of the tray.

In these tests, the film obtained in this Example 1 has received theassessment 5 throughout the testing time.

Films produced following the same procedure as described in this Example1 but adding only two of the additives (i), (ii) and (iii) have receivedassessments that are lower in the absolute and/or have not shown acomparable activity of such long duration or of such immediate evidence.

Films having a very good anti-fog activity may be obtained followingsubtantially the same procedure as described in this Example 1, butreplacing the polymers or their blends according to the teaching of theEuropean patent EP-B-217.596, or following substantially the sameprocedure of this Example 1 but changing the total amount of theadditives (i), (ii) and (iii) and/or their ratio so that the mixture of(i) and (ii) is present in a percentage quantity by weight on the totalweight of the film of from about 0.5 to about 1, and that (iii) ispresent in a percentage quantity of from about 1 to about 2.

Films having anti-fog activities according to the present invention canalso be obtained by following substantially the same procedure of thisExample 1 but using a mixture of glycerol mono- and di-stearate insteadof the mixture of glycerol mono- and di-oleate.

Further films that can suitably be additivated with about 1 part byweight of a mixture of glycerol mono- and di-oleate and about 2 parts byweight of ethers of polyoxyethylene with fatty alcohols having from 12to 14 carbon atoms, in a percentage quantity by weight on the totalweight of the film of from about 1.25 to about 2, are disclosed asstructures I, II and III in the U.S. Pat. No. 4.551.380.

In this case the incorporation of anti-fog additives can be made in thefilm composition of any one of the layers.

We claim:
 1. A heat shrinkable thermoplastic film having at least oneexternal layer comprising a polyolefin, the film containing between 1%and 5%, by weight of the film, of a composition comprising:a) a glycerolmono-ester of a saturated or unsaturated fatty acid having between 8 and20 C; b) a glycerol di-ester of a saturated or unsaturated fatty acidhaving between 8 and 20 C; and c) at least one polyoxyethylene ether ofa fatty alcohol having between 8 and 20 C;wherein the weight ratio ofthe combined esters a) and b) to the weight of the ether c) is between2:1 and 1:4.
 2. The film of claim 1 wherein the composition comprisesbetween 1.25% and 3.5% by weight of the total film.
 3. The film of claim1 wherein the weight ratio between the ester a) and the sum of theesters a) and b) is between 0.25:1 and 0.75:1.
 4. The film of claim 1wherein the weight ratio between the ester a) and the sum of the estersa) and b) is between 0.4:1 and 0.6:1.
 5. The film of claim 1 wherein thesaturated or unsaturated fatty acid of ester a) or ester b) has between12 and 20 C.
 6. The film of claim 1 wherein the saturated or unsaturatedfatty acid of ester a) or ester b) comprises oleic acid.
 7. The film ofclaim 1 wherein the ether c) is essentially the reaction product of amixture of fatty alcohols having between 8 and 20 C with ethylene oxide.8. The film of claim 7 wherein the fatty alcohol has between 8 and 18 C.9. The film of claim 7 wherein the mixture of fatty alcohols comprises amixture of lauryl alcohol and myristyl alcohol.
 10. The film of claim 1wherein the polyolefin comprises:a) ethylene/vinyl acetate copolymer;and b) a copolymer or terpolymer of ethylene and one or morealpha-olefins.
 11. The film of claim 10 comprising:a) an internal layercomprising a blend of ethylene/vinyl alcohol co-polymer and polyamide;b) two intermediate adhesive layers; and c) two external layers eachcomprising a blend of linear low density polyethylene, linear mediumdensity polyethylene, and ethylene/vinyl acetate copolymer.
 12. The filmof claim 1 wherein the film is crosslinked.
 13. The film of claim 1wherein the film is oriented.
 14. The film of claim 13 wherein the filmis heat shrinkable.
 15. A method for packaging a product comprising:a)placing the product in a tray; b) wrapping the trayed product in a heatshrinkable thermoplastic film having at least one external layercomprising a polyolefin, the film containing between 1% and 5%, byweight of the film, of a composition comprising:i) a glycerol mono-esterof a saturated or unsaturated fatty acid having between 8 and 20 C; ii)a glycerol di-ester of a saturated or unsaturated fatty acid havingbetween 8 and 20 C; and iii) at least one polyoxyethylene ether of afatty alcohol having between 8 and 20 C;wherein the weight ratio of thecombined esters a) and b) to the weight of the ether c) is between 2:1and 1:4; c) sealing or clipping the film; and d) shrinking the film suchthat a surface of the external layer facing the product is not incontact with the product.
 16. A package comprising:a) a product disposedon a tray; and b) a film, in which the trayed product is wrapped andshrunk, having at least one external layer comprising a polyolefin, thefilm containing between 1% and 5%, by weight of the film, of acomposition comprising:i) a glycerol mono-ester of a saturated orunsaturated fatty acid having between 8 and 20 C; ii) a glyceroldi-ester of a saturated or unsaturated fatty acid having between 8 and20 C; and iii) at least one polyoxyethylene ether of a fatty alcoholhaving between 8 and 20 C.